1. Field of the Invention
The present invention relates to image forming equipment and, more particularly, to a multi-tone laser beam writing apparatus capable of rendering multiple tones by a single dot.
2. Discussion of the Background
Some modern image forming equipment such as digital copiers, laser beam printers and facsimile machines have a capability of rendering an image in multiple tones. For example, a digital copier with such a capability reproduces an image in multiple tones by using dither and renders, among others, a photographic image smoothly. Increasing the dither would enhance the reproduction of tones, but it would lower the resolution. While this dilemmatic situation may be eliminated by increasing the writing density, such an approach increases the cost and impedes high-speed processing. It follows that, whether the document be a text document or a photographic document, both the high resolution and the high reproducibility of tones are achievable if data can be written in multiple tones by a single dot.
Generally, multi-tone laser beam writing is implemented by either one of two different systems, i.e., a pulse width modulation (PWM) system and an analog modulation system. The PWM system is disclosed in, for example, Japanese Patent Laid-Open Publication (Kokai) No. 125338/1988. PWM systems heretofore proposed have a drawback in that the output PWM signal is not accurate because the image signal contains noise and because it is not easy to form a pattern signal and to synchronize the image signal and pattern signal to each other. The analog modulation system is taught in, for example, Japanese Patent Laid-Open Publication No. 184773/1988. A problem with the conventional analog modulation system is that the number of tones cannot be increased without increasing the component parts and elements of a voltage applying device and without complicating the control. In this connection, to provide 2.sup.n tones, n constant current sources are needed. Even though light issuing from a semiconductor laser may be controlled in quantity on the basis of a great number of tones, it is likely that an image cannot be printed out in densities matching the quantities of output light of a semiconductor laser due to the aging of a photoconductive drum and toner included in the laser beam printer. The analog modulation system is disadvantageous for the above reasons, although it is capable of providing an infinite number of tones. While the PWM signal produced by the PWM system may be subjected to analog modulation, the accuracy of the PWM signal is not high enough to achieve desirable tone reproducibility.